Samoa's government has said it suspects that the measles virus has caused seven deaths, since the outbreak was declared in October.
As of Monday, 628 suspected measles cases had been recorded with 48 confirmed so far.
Auckland University's Helen Petousis-Harris said the disease was at the stage where it was now difficult to stop.
She said the deaths were a tragedy.
"How it can be stopped, I don't know, it's like a wildfire that is burning now out of control, so how you can contain something like that is enormously difficult."